OK, there are a number of different variations.
You can use metal; wood; bolts, washers and wing nuts; bent over nails. It looks like they all work.
It can be simple. Four screws, a piece of wood, and a boxcutter blade.
Tools needed were a screwdriver, a boxcutter and a couple of clamps to hold it.
A boxcutter was used to cut the end off the bottle but the boxcutter blade could have been used for this. It was a fairly ragged cut so it didn't work well at first. I used the little scissors on a swiss army knife to tidy the cut.
Tip number 1: make it a smooth initial cut.
Initially I had the blade loosely screwed down. I was thinking of using downward pressure on the bottle to hold it in place. This doesn't work very well.
Tip number 2: make sure the blade is fixed nice and solid.
One screw is used to hold the blade down (far right). There is a small piece of folded over plastic between the blade and the wood (not visible). This is to lift the blade up away from the wood. I found this isn't necessary but may help in getting the desired string thickness if using a different design.
The second screw is under the blade (in behind the screw in the centre of the bottle). This holds the other end of the blade up to give the desired thickness to the cut plastic. The blade is held down on this by the tension from the first screw.
The screw in the centre of the bottle stops the blade from moving back. The bottle is spun clockwise (looking from the top) as the cutting edge is on the far side of the blade.
The two screws at the edges of the bottle are guides. The placement of the right hand one determines the string thickness. Move this screw to the left for a thicker string or to the right for thinner string. This aligns the bottle on the blade where the gap is the desired thickness.
The other guide screw is placed at the other side of the bottle to stop it wobbling around too much.
In practice, when the initial cut was ragged only small sections of string would cut off. Whenever a dip in the plastic came around the string just tapered off and ended.
Making the initial cut smooth fixed this.
Then when the blade wasn't firmly screwed down it would wiggle up and down regardless of the downward pressure on the bottle. So the string was again only small pieces.
Once these two things were fixed it went really well.
To get it started press gently down on the top of the bottle and spin it. It is surprisingly easy to do.
I found the string wouldn't come through initially like in the videos, probably because of the roughness of the wood or the lack of a channel to act as a string guide. Easily fixed, just spin the bottle back, grab the short piece of existing string and feed it under the blade. Spin the bottle back in place and once set, pull on the string. It does work as easily as you see on the videos!
Sometimes you can release the bottle and just pull on the string but this can lead to erratic results, either with the string tapering off and ending or the string width getting wider as the bottle tends to fold under the blades as it is cutting. Best results were from gently holding the bottle in place with a little downward pressure. If the top of the bottle remains in roughly the same place when spinning this gives the most consistent results. I guess this is why some of the designs use a rod through the top of the bottle.
Tip number three: don't let the bottle wobble around.
The results did vary a bit, sometimes the string would cut off short a few times before it went well. And the width of the string varied as well. I suspect this is due to the rough nature of the build. Using a hacksaw blade to put a guide channel in the wood would probably help. As the bottle angle varied the wall of the bottle would bend over a little as it went into the blade giving wider string, then when the bottle was more upright the string was thinner. Using a piece of wood which is as wide as the bottle will help a lot I think as it will stop the bottle tilting as much.
Maybe a few more 'guide' screws around the circumference of the bottle would help too.
The bottle used was a Schweppes 2 litre soft drink bottle. It has smooth sides except for a ridge halfway up which I suspect is for strengthening. The cutter didn't like the ridge and I ended up taking the bottle out and cutting the ridge away and then starting again.
And when the curve at the top of the bottle was reached the string became much wider as the plastic angle became excessive. No useful string from it but fun to do! Designing the cutter so it always maintained a constant plastic angle would possibly allow use of all the plastic up to the lid.
I will try a more elaborate design and once it works well will try some of the non-smooth sided bottles to see if they can be used.
LATER:
I tried a second wider one hoping the extra width would help to stabilise the bottle. Mixed results. It looks like controlling the way the plastic comes onto the blade is more important than having the bottle in a set position. This would be why other designs on the web don't require the bottle to be held.
Similar setup. The blade is screwed down at one end and rests on a screw to give a space between the blade and the base. This one was setup for anticlockwise rotation, just because that is how it happened!
The first try had the blade parallel to the wood where it cuts the plastic. This made it really difficult to start the string. The bottle would ride over the top of the blade and never bite. Starting the string with a knife and feeding it under didn't help much. The string would just taper off.
Ended up using an extra screw to pull down the back of the blade so the edge was at the highest point. Much better but the blade needs to be locked solid in this position. If it moves it either cuts the string or alters the string thickness.
Also using only three of the guide screws, letting the bottle run inside the far one gave much better results, more uniform string and easier cutting. On closer inspection, I think this is because the blade was cutting the bottle at an angle. A 45 degree blade angle was mentioned in one of the webpages I looked at. This may be one of the features that makes them work well.
The results.
The plastic on the left is from the narrow design. Many short strings initially until the blade was locked in place firmly. Then it worked OK but there was a fair bit of variation in string width.
The centre string is about 11m long and is from the wider design. Again a fair bit of variation in string width but nowhere near like the first design. I would consider this string to be usable. Width varied from 3 to 9mm. This string also has the ridge at the middle of the bottle in it so it isn't necessary to cut this out. The ridge will cut fine with the right design.
The last lot is also from the wider design. I slightly changed the design by locking the blade in place with another screw. It changed something but I'm not sure what. The string was very uniform but is was difficult to keep going. The longest string was 3 m and to get this the bottle had to be held in the one position and the string pulled through very gently and precisely.
Both other strings could be pulled through without much care.
There were many other short pieces as well.
More to come.